You just must keep trying. It is possible (without cheats).
If it is still irritating - there's the other way you can cheat. Make your own barricade with:ent_create item_ammo_crate
Load a save just before you turn off that generator. Do not turn it off. Go back and create your own barricade (between desk and wall, so NPC can't go near that ventilation). Go back, turn off the generator, stick with your group.
I did it without cheats. Made a barricade with every prop I could find.